Emitters of a backside contact solar cell

ABSTRACT

A system and method of patterning dopants of opposite polarity to form a solar cell is described. Two dopant films are deposited on a substrate. A laser is used to pattern the N-type dopant, by mixing the two dopant films into a single film with an exposure to the laser and/or drive the N-type dopant into the substrate to form an N-type emitter. A thermal process drives the P-type dopant from the P-type dopant film to form P-type emitters and further drives the N-type dopant from the single film to either form or further drive the N-type emitter.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a PNjunction between P-type and N-type diffusion regions. Solar radiationimpinging on the surface of, and entering into, the substrate of thesolar cell creates electron and hole pairs in the bulk of the substrate.The electron and hole pairs migrate to P-type diffusion and N-typediffusion regions in the substrate, thereby creating a voltagedifferential between the diffusion regions. The diffusion regions areconnected to conductive regions on the solar cell to direct anelectrical current from the solar cell to an external circuit. In abackside contact solar cell, for example, both the diffusion regions andthe interdigitated metal contact fingers coupled to them are on thebackside of the solar cell. The contact fingers allow an externalelectrical circuit to be coupled to and be powered by the solar cell.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a solar cell laser system in accordancewith an embodiment.

FIG. 2 is a flow chart of an embodiment of a method, technique, andprocess for forming emitters of a backside contact solar cell with acombination of a laser doping and a thermal doping, according to oneembodiment.

FIGS. 3A-3F illustrate cross-sectional views of forming emitters of abackside contact solar cell, according to some embodiments.

FIG. 4 is a schematic representation of an embodiment of a method,technique, and process for forming an emitter of a backside contactsolar cell with lasers, according to one embodiment.

FIG. 5 illustrates a cross-sectional view of an emitter of a backsidecontact solar cell with lasers, according to some embodiments.

FIG. 6 is a flow chart of an embodiment of a method, technique, andprocess for forming emitters of a backside contact solar cell thermally,according to one embodiment.

FIGS. 7A-7B illustrate cross-sectional views of forming emitters of abackside contact solar cell thermally, according to some embodiments.

FIG. 8 is a flow chart of another embodiment of a method, technique, andprocess for forming both emitters of a backside contact solar cell bylaser doping, according to one embodiment.

FIG. 9 illustrates a cross-sectional view of emitters of a backsidecontact solar cell, according to some embodiments.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter of theapplication or uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” dopant source does not necessarily imply that this dopant sourceis the first dopant source in a sequence; instead the term “first” isused to differentiate this dopant source from another dopant source(e.g., a “second” dopant source).

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

“Inhibit”—As used herein, inhibit is used to describe a reducing orminimizing effect. When a component or feature is described asinhibiting an action, motion, or condition it may completely prevent theresult or outcome or future state completely. Additionally, “inhibit”can also refer to a reduction or lessening of the outcome, performance,and/or effect which might otherwise occur. Accordingly, when acomponent, element, or feature is referred to as inhibiting a result orstate, it need not completely prevent or eliminate the result or state.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Although much of the disclosure is described in terms of solar cells forease of understanding, the disclosed techniques and structures applyequally to other semiconductor structures (e.g., silicon wafersgenerally).

In the following description, numerous specific details are set forth,such as specific process flow operations, in order to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to one skilled in the art that embodiments of the presentdisclosure may be practiced without these specific details. In otherinstances, well-known fabrication techniques, such as lithographytechniques, are not described in detail in order to not unnecessarilyobscure embodiments of the present disclosure. Furthermore, it is to beunderstood that the various embodiments shown in the figures areillustrative representations and are not necessarily drawn to scale.

Interdigitated back contact (IBC) solar cells require that both P and Ntypes of emitters are formed on the backside of the wafer, in a specificpattern. This patterning and formation of the two emitters results inmuch of the cost for forming the IBC cell. Reduction of emitterformation cost is desirable to make an IBC cell process more costeffective.

Accordingly, a method is described for patterning the dopants to form aninterdigitated back contact (IBC) solar cell. IBC solar cells eliminatecontact-related shading losses by putting both contacts on the rear ofthe cell. In one embodiment, the method includes blanket deposition oftwo dopant films, a Boron Silicate Glass (BSG) film followed by aPhosphorous Silicate Glass (PSG) film. A laser may be applied ordirected such as impinged to pattern an N-type dopant, by mixing the PSGand BSG films where they are exposed to the laser to form a boro phosphosilicate glass (BPSG) film and/or drive Phosphorous into a solar cellsubstrate to form an N-type emitter. After the laser patterning andlaser diffusion, the solar cell is subjected to a thermal anneal, whichserves to drive the Boron dopant from the non-lased BSG regions to formP-type emitters and drive Phosphorous dopant from the lased BPSG regionsto either form or further drive the N-type emitter. This thermal annealcan also serve to repair any damage to the substrate from the laserpatterning or blanket deposition step. The method relies on laserpatterning design rules, which are tighter and more precise than otherpatterning methods, particularly screen-printing. Because the dopantfilms are deposited as a blanket film stack without any patterning stepin-between, they can be deposited in a single tool. This process canremove the need for a mask and an etch process, removing a wet tool foroxide etch from the fabrication line.

This specification first describes a laser system used to form anexample solar cell, followed by a more detailed explanation of variousembodiments of forming emitters of solar cells with a laser process.Various examples are provided throughout.

FIG. 1 schematically illustrates a solar cell laser system 100 inaccordance with an embodiment. In the example of FIG. 1, the lasersystem 100 includes a laser source 102 and a laser scanner 104. Althoughillustrated for descriptive purposes, the drawings are not to scale andsome items may be omitted for clarity. The arrangement shown isrepresentative of the components and may or may not reflect their truepositioning relative to one another in some embodiments. The laserscanner 104 may comprise a galvanometer laser scanner.

In operation, the laser source 102 generates laser pulses 103 at apredetermined wavelength, in accordance with a configuration 101. Theconfiguration 101 may comprise switch/knob arrangements,computer-readable program code, software interface settings, and/orother ways of setting the configurable parameters of the laser source102. The configuration 101 may set the pulse repetition rate, number ofpulses fired per repetition, pulse shape, pulse amplitude, pulseintensity or energy, and other parameters of the laser source 102. Thelaser scanner 104 scans the laser pulses 103 across a solar cell 105being fabricated.

Generally, a film stack consisting of a BSG and a PSG layer as twodopant films is formed above a wafer, with the BSG film as the film incontact with an underlying silicon substrate. The substrate could beeither a crystalline silicon or a polysilicon substrate. By using thelaser system 100, a laser is used to pattern the substrate, lasing theareas which will comprise an N-type emitter. Upon lasing, the film stackundergoes a mixing where a BPSG film is generated from the composite ofthe BSG and PSG films. During the lasing, an amount of the Phosphorousand/or Boron dopants may be diffused into the silicon substrate.Phosphorous preferentially dopes over Boron out of the BPSG film, andthe N-type emitter can be created from the diffusion. After lasing thepattern into the film stack, the wafer can be subjected to a thermalanneal, which functions to drive Boron from the BSG film into thesubstrate to form the P-type emitter. During this thermal anneal,Phosphorous is also driven from the BPSG film to form or further drivethe diffusion for the N-type emitter. The anneal process can also serveto repair any damage to the silicon substrate caused by the laserpatterning step. Thus by this process both emitters for an IBC solarcell are created.

Referring to FIG. 2, a flowchart 200 is shown that represents operationsin a method of forming P-type and N-type emitters for a backside contactsolar cell 360 (FIG. 3F) such as an IBC solar cell, in accordance withan embodiment. FIGS. 3A-3F illustrate cross-sectional views of variousstages in the fabrication of the backside contact solar cell 360,corresponding to operations of flowchart 200, in accordance with anembodiment of the present invention. In this example, the mentionedprocess steps are performed in the order shown. In other examples, theprocess steps can be performed in other orders. It is to be noted thatother process steps not necessary for understanding are omitted in theinterest of clarity.

In addition, in some embodiments, the process can include fewer than allthe illustrated steps in FIG. 2. The various process steps performed inconnection with the method shown in the flowchart 200 may be performedby software, hardware, firmware, or any combination thereof. Forillustrative purposes, the following description of the method shown inthe flowchart 200 may refer to elements mentioned above in connectionwith FIGS. 3A-3F. In practice, portions of the method shown in theflowchart 200 may be performed by different elements of the describedsystem, e.g., the solar cell laser system 100. It should be appreciatedthat the method shown in the flowchart 200 may include any number ofadditional or alternative process steps, the method shown in theflowchart 200 may be incorporated into a more comprehensive procedure orprocess having additional functionality not described in detail herein.

Referring to operation 202 of the flowchart 200, and to correspondingFIGS. 3A-3B, a method of forming emitters for the backside contact solarcell 360 includes forming a homogenous stack 300 (FIG. 3B) of at leasttwo dopant material source layers above a solar cell substrate 302 (seeFIG. 3A) of the solar cell 360. In an embodiment, the substrate 302 is abulk single-crystal substrate, such as an N-type doped singlecrystalline silicon substrate or N-type silicon wafer. However, in analternative embodiment, the substrate 302 may include a polycrystallinesilicon layer disposed on a global solar cell substrate.

As illustrated, FIG. 3A shows a first dopant material source layer 304of the stack 300 and it has dopants of a first impurity conductivitytype such as P-type, e.g., Boron. For example, a P-type dopant source ofBoron with a dopant concentration level of approximately in a range of1-5% wt as the BSG film may be used to form a P-type emitter in the Siwith 1E18 atom/cm3-1E21 atom/cm3 concentration.

FIG. 3B shows a second dopant material source layer 306 over the firstdopant material source layer 304 and it has dopants of a second impurityconductivity type being of opposite polarity from dopants of the firstimpurity conductivity type such as N-type, e.g., Phosphorous. Forexample, an N-type dopant source that comprises Phosphorus with a dopantconcentration level of approximately in a range of up to 10% wt as thePSG film may be used to form an N-type emitter in the Si with 1E18atoms/cm3-1E21 atoms/cm3 concentration. Although N-type dopants asPhosphorous and P-Type dopants as Boron are described, other donor andaccepter dopants can be used in other embodiments.

As shown, the stack 300 is deposited on a backside surface of thesubstrate 302. As illustrated, FIG. 3A shows the solar cell substrate302 having a backside 305 and a front side 306. There are a plurality ofP-type diffusion regions and N-type diffusion regions in a solar cellbut only one of each is shown as being fabricated in the followingexample for clarity of illustration.

In certain embodiments, including the illustrated embodiment, the stack300 includes two dopant material source layers, the first dopantmaterial source layer 304 and the second dopant material source layer306. In other embodiments, however, more layers including dopantmaterial source layers can be included in the stack 300. Thus, otherembodiments of the stack 300 can be used in the techniques describedherein.

In an embodiment, forming the stack 300 of at least two dopant materialsource layers includes depositing a first dopant source film of a firstimpurity conductivity type above the substrate 302 of the solar cell anddepositing a second dopant source film of a second impurity conductivitytype above the first dopant source film. The first and second dopantsource films may be formed by Chemical Vapor Deposition (CVD). In anembodiment, the first dopant material source layer 304 is composed ofboron silicate glass (BSG) and has a thickness approximately in therange of 400-500 Angstroms (e.g., films down to ˜200 Angstroms). In oneembodiment, the total deposited thickness, including an un-doped cappinglayer may be in a range of 1000-3000 Angstroms.

In a specific embodiment, the BSG layer is formed by chemical vapordeposition as a uniform, blanket layer. In a particular such embodiment,the BSG layer is formed by a chemical vapor deposition technique suchas, but not limited to, atmospheric pressure chemical vapor deposition(APCVD), plasma-enhanced chemical vapor deposition (PECVD), low-pressurechemical vapor deposition (LPCVD), or ultra-high vacuum chemical vapordeposition (UHVCVD). The concentration of Boron (B) in the BSG layer isat levels in a range of ˜4% to ˜1-2%. It should be noted that thediscussion of the formation of the BSG layer is not intended to belimiting as to the scope of the invention described herein since adoping layer, e.g., the first dopant material source layer 304 couldalso be formed using other materials (e.g., spin-on or printed dopants)that also provide an amount of a dopant material to the surface of thesubstrate 302 during one or more laser and/or thermal processing stepswithout deviating from the basic scope of the embodiment describedherein.

In one embodiment, the second dopant material source layer 306 comprisesphosphorus silicate glass (PSG) grown on the surface of the boronsilicate glass (BSG) and has a thickness approximately in the range of400-500 Angstroms (e.g., films down to ˜200 Angstroms). In a specificembodiment, the PSG layer is formed by chemical vapor deposition as auniform, blanket layer. In a particular such embodiment, the PSG layeris formed by a chemical vapor deposition technique such as, but notlimited to, atmospheric pressure chemical vapor deposition (APCVD),plasma-enhanced chemical vapor deposition (PECVD), low-pressure chemicalvapor deposition (LPCVD), or ultra-high vacuum chemical vapor deposition(UHVCVD). The concentration of Boron (B) in the PSG oxide layer is atlevels in a range of ˜4% to ˜1-2%. It should be noted that thediscussion of the formation of the PSG layer is not intended to belimiting as to the scope of the invention described herein since adoping layer, e.g., the second dopant material source layer 306 couldalso be formed using other materials that also provide an amount of adopant material to the surface of the substrate 302 during one or morelaser and/or thermal processing steps without deviating from the basicscope of the invention described herein.

Referring to operation 204 of the flowchart 200, and to correspondingFIG. 3C, forming of a heterogeneous single dopant material source layer310 such as a charge-neutral layer containing both P-type dopants andN-type dopants sourced from both the first and second dopant materialsource layers 304, 306 (FIG. 3B) is shown. As illustrated, FIG. 3C showsa first laser beam 312 directed on a first portion 315 of the stack 300to form the single dopant material source layer 310. The first laserbeam 312 may be generated from the laser source 102 as shown in FIG. 1.A first laser profile of the first laser beam 312 may be tuned as theconfiguration 101 (FIG. 1) to mix dopants of both the P and N impurityconductivity types from the first and second dopant material sourcelayers 304, 306 into the single dopant material source layer 310. It isto be understood that a laser profile, such as the first laser profileencompasses use of a particular laser tool, a laser beam of a particularcharacteristic or uses certain specific settings based on a laserprocess that may be deployed in an embodiment.

Consistent with one embodiment, the single dopant material source layer310 is a boro phospho silicate glass (BPSG) layer. The Boron content ofthe BPSG may be less than the Phosphorus content. For example, thepercent composition of Boron may range from 0.1% to 5% and the percentcomposition of Phosphorus may range from 5% to 8% in the BPSG layer. Asa particular example, the BPSG layer may have (a) 5% boron and 5%phosphorus, (b) 3% boron and 5% phosphorus, or (c) 5% boron and 8%phosphorus. The percent composition of Boron and Phosphorus in the BPSGlayer may vary depending on the process. It should be noted that thediscussion of the formation of the BPSG layer is not intended to belimiting as to the scope of the invention described herein since adoping layer, e.g., the single dopant material source layer 310 couldalso be formed using other materials that also provide an amount of adopant material to the surface of the substrate 302 during one or morethermal processing steps without deviating from the basic scope of theembodiment described herein.

One example of a laser for the first laser beam 312 is a diode-pumpedsolid state (DPSS) ultra-violet (UV) laser of 1064 nm wavelength whichin one embodiment is changed to 532 nm using a frequency dubbler and/orchanged to 355 nm using a tippler. The UV laser can be applied to thestack 300 so that the single dopant material source layer 310 forms as a(BPSG) layer. Other UV laser such as an excimer laser may be used.

Further referring to operation 204 of the flowchart 200, and tocorresponding FIG. 3C, by using the first laser beam 312 the substrate302 may be patterned to form a first emitter 318 of the solar cell fromthe dopants of N-type type that are available in the single dopantmaterial source layer 310. The first laser beam 312 diffuses the dopantsof N-type into a first portion 320 of the substrate 302 underneath thesingle dopant material source layer 310 at a first dopant concentrationlevel (1E18 atom/cm3-1E19 atom/cm3) based on the N-type dopants such asapproximately 1-10% wt present in the second dopant material sourcelayer 306. Although the first emitter 318 is illustrated as a narrowregion, in some embodiments, the first emitter 318 can be formed as bigas a P-type emitter. FIGS. 3C-3D illustrate a cross-sectional view of anexemplary N-type emitter.

One example of a laser for the first laser beam 312 is a UV laser todrive the N-type dopants from the single dopant material source layer310, i.e., the (BPSG) layer into the substrate 302. The UV laser can beapplied to the single dopant material source layer 310, i.e., the (BPSG)layer so that the first emitter 318 is formed. Alternatively a greenlaser may be used to heat the underlying substrate, which would enabledopant diffusion in the Si.

Referring to operation 206 of the flowchart 200, and to correspondingFIG. 3D, patterning of the substrate 302 is shown to form a secondemitter 325 of the solar cell 360 from the dopants of P-type in thefirst dopant material source layer 304 of the stack 300 adjacent to thefirst emitter 318. The second emitter 325 being of P-type is differentthan the first emitter 318 being of N-type.

Referring to operation 208 of the flowchart 200, and to correspondingFIG. 3D, to form the second emitter 325, the dopants of P-type may bethermally driven from a second portion 327 of the stack 300 of the firstand second dopant material source layers 304, 306 into a second portion330 of the substrate 302 adjacent to the single dopant material sourcelayer 310. For diffusing dopants, a thermal anneal or a high-temperaturedrive step can be performed, in one embodiment, to diffuse dopants fromthe first dopant material source layer 304 to form a P-type diffusionregion in the substrate 302. The thermal anneal or high-temperaturedrive step may be performed for about a time in a range of 30-60 minutes(e.g., 30 minutes) at a temperature range between 600° C. and 1100° C.(e.g., 950° C.). The furnace temperatures can correspond to any selectedfrom the range between 600° and 1100° C., such as a lower temperature of700° C. and an upper temperature of 900° C. As can be seen from FIG. 3D,the second portion 330 of the stack 300 is different than the firstportion 315 of the stack 300 and the second portion 330 of the substrate302 is different than the first portion 320 of the substrate 302.

Consistent with one embodiment, a P-type dopant source that comprisesBoron may be used to form a P-type diffusion region of the secondemitter 325. Likewise, an N-type dopant source that comprises Phosphorusmay be used to form an N-type diffusion region of the first emitter 318.A dopant source is a source of charge carrier impurity atoms for asubstrate such Boron is for a silicon based substrate. For example, inone embodiment, the charge carrier impurity atoms are N-type dopants,such as but not limited to phosphorus dopants. In another embodiment,the charge carrier impurity atoms are P-type dopants, such as but notlimited to boron dopants. In a back-contact solar cell, such as thesolar cell 360, with interdigitated N-type and P-type diffusions in thesubstrate 302 there is a PN junction that may be formed within thesubstrate 302 at an interface between the two diffusions.

In one embodiment, the P-type diffusion region and N-type diffusionregion are active regions. Conductive contacts may be coupled to theactive regions and separated from one another by isolation regions,which may be composed of a dielectric material. In an embodiment, thebackside contact solar cell 360 further includes an anti-reflectivecoating layer (e.g., dielectric) disposed on a light-receiving surface,such as on a random textured surface of the solar cell.

The solar cell 360 may further include conductive contacts formed onemitter regions which are formed in the substrate 302, in accordancewith an embodiment. A first electrically conductive contact such as afirst metal contact finger may be disposed in a first contact openingand may be coupled to the N-type diffusion region. A second electricallyconductive contact such as a second metal contact finger may be disposedin a second contact opening and may be coupled to the P-type diffusionregion. In an embodiment, a first metal contact finger and a secondmetal contact finger are back contacts for the back-contact solar cell360 and are situated on a surface of the solar cell 360 opposing a lightreceiving surface of the solar cell 360. The “fingers” may be made usingmasks and etch or according to other techniques.

Referring to operation 210 of the flowchart 200, and to correspondingFIGS. 3E-3F, metal contacts may be formed to the first and secondemitters 318, 325, respectively. In FIG. 3E, according to oneembodiment, optionally insulator layers 335 may be formed over portionsof the stack 300 that lie over the P-type diffusion regions of thesecond emitters 325. The insulator layers 335 are formed byscreen-printing in one embodiment. Alternatively, these can be formed byink-jet printing or other low-cost printing technique. Accordingly,insulator layers 335 may comprise polyimide or other dielectric that maybe formed by screen-printing or ink-jet printing. First contact holes340 are defined in the insulator layers 335 to allow subsequently formedfirst metal contacts 345 (See FIG. 3F) to electrically contact theP-type diffusion regions of the second emitters 325.

In FIG. 3F, portions of the stack 300 under the first contact holes 340are etched away. Likewise, portions of the stack are etched away to formsecond contact holes 350. The first metal contacts 345 are formed overthe insulator layers 335 and through the first contact holes 340 tocreate electrical connections between the first metal contacts 345 andthe P-type diffusion regions of the second emitters 325. Second metalcontacts 355 are formed over the stack 300 and through the secondcontact holes 350 to create electrical connections between the secondmetal contacts 355 and the N-type diffusion region of the first emitter318. The first contact holes 340 and the second contact holes 350 do nothave to be made by a laser but could be made with a wet etch process andin one embodiment go all the way to the first and second emitters 318,325, respectively, and do not stop in any of the oxide layers.

The backside contact solar cell 360 as shown in FIG. 3F, and the othersolar cells disclosed herein, may also be fabricated using thefabrication steps disclosed above. Other fabrication techniques forfabricating the solar cell structures of the backside contact solar cell360 may also be used without detracting from the merits of the presentdisclosure.

Each embodiment of a backside contact solar cell can have a differentform. One example embodiment is illustrated in FIG. 3F with adjacentfirst and second emitters 318, 325. As used herein, diffusion regionsand emitters are used interchangeably to refer to doped regions of thesubstrate 302 of a solar cell. In certain embodiments, including theillustrated embodiment, the backside contact solar cell 360 includes thefirst emitter 318 surrounded by two second emitters 325. In otherembodiments, however, more or fewer emitters can be included in a solarcell, resulting in variations in the size and shape. Thus, otherembodiments of emitters can be used in the solar cells described herein.For example, the number and location of the emitters can vary betweenembodiments. The first and second emitters 318, 325 are illustrated ashaving a rectangular shape, although any other desired shape or size canbe used, such as circular, irregular, and so on. In certain embodiments,including the illustrated embodiment, the dopant concentration levelsare similar in the first and second emitters 318, 325, while in otherembodiments, they may be different from one another, depending upon aparticular embodiment of the solar cell.

Use of laser doing, in one embodiment, provides more precise alignment,e.g., 10 microns vs. 100 microns available from other patterningprocesses. And another advantage is that an entire panel of solar cellscan be laser treated at once as compared to one solar cell beingfabricated by standard patterning techniques.

The first and second dopant material source layers 304, 306 thicknessesmay vary depending on the laser and thermal processes, to give anoptimal dopant selection. For example, the first dopant material sourcelayer 304, e.g., the BSG film may be thick enough so that thePhosphorous does not drive through the BSG film during the thermal drivestep. The amount of Boron in the BSG film and the amount of Phosphorousin the second dopant material source layer 306, e.g., the PSG film maybe selected to achieve a desired doping of the second emitter 325, e.g.,the P-type emitter and the first emitter 318, e.g., the N-type emitter,respectively. While the Boron doping may be controlled by the Boronconcentration in the BSG film, the BSG film thickness and the thermalanneal step, the Phosphorous doping may be controlled by the Phosphorousconcentration in the PSG film, the laser process, the Boronconcentration in the BSG film (due to the BPSG formation), and thethermal anneal step. The thermal process may be be optimized for both ofthe dopant diffusions and the anneal removal of laser damage to theemitter, as well as combined into a front solar cell surface passivationstep if desired.

Overall, the process flow of the flowchart 200 as shown in FIG. 2 mayresult in simplification of a solar cell fabrication process bypatterning the BSG and PSG layers independently, removing a mask stepand an etch step or combining the BSG and PSG depositions into a singlestep/tool. The process flow of the flowchart 200 may also allow forbetter design rules, which may decrease the pitch of the solar cell andthereby increase cell efficiency while decreasing cost.

Referring to FIG. 4, a flowchart 400 is shown that represents operationsin a method of forming a selective N-type emitter for a backside contactsolar cell, in accordance with an embodiment. As such, in oneembodiment, the selective N-type emitter enables a relatively betterpassivation with a lower level of doping outside of the metal contactregion, and better contact resistance with higher doping within thecontact region.

FIG. 5 illustrates a cross-sectional view of a stage in the fabricationof a backside contact solar cell, corresponding to operations offlowchart 400, in accordance with an embodiment of the presentinvention.

The embodiments shown in FIGS. 4 and 5 can include all or some of themethod steps shown in FIGS. 2 and 3A-3F. However, in order to form aselective N-type emitter, an additional process step can be introducedeither after step 206 or step 208. As illustrated, an additional step isbeing introduced after the method step of 208 of FIG. 2 having thecorresponding cross-sectional view FIG. 3D. This additional step isdescribed next as the first method step in FIG. 4.

Referring to operation 402 of the flowchart 400, and to correspondingFIG. 5, a method of forming a trenchless N-type emitter, i.e., a firstemitter for a backside contact solar cell includes directing a secondlaser beam 512 on a region 502 of the first portion 315 of the stack 300to diffuse the dopants of N-type impurity conductivity type at a seconddopant concentration level by using the second laser beam 512. Asillustrated, the second dopant concentration level is higher than thefirst dopant concentration level of the N-type dopants in the remainingfirst portion 315 of the stack 300 (see FIG. 5). As illustrated, FIG. 5shows a selective emitter embodiment of the first emitter 318. Onepotential benefit of forming the emitters using such an approach is thepossibility of forming a sharp diffusion boundary in the Si, between thepositive and negative regions, which would allow a lower trenchrecombination. Typical thermal diffusions result in a large boundarylayer between positive and negative regions, which results insignificant recombination and lower efficiency. High-efficiency solarcell design often requires the formation of a trench to remove this areaand prevent recombination, however, these extra steps add cost andcomplexity. Use of lasers to form sharp junctions opens the space ofhigh-efficiency, cost-effective process.

Referring to operation 404 of the flowchart 400, metal contacts may beformed to the first and second emitters 318, 325, respectively in thesame manner as shown in FIGS. 3E-3F.

Another embodiment of forming the first and second emitters 318, 325 isshown in FIG. 6. For the ease of understanding of this embodimentrelative to the embodiment shown in FIG. 2, the steps 204 and 206 ofFIG. 2 are referred to again next in the context of FIG. 6. The specificembodiment shown in FIG. 6 can include all the method steps shown inFIGS. 2 and 3A-3F except that the laser step 204 is altered and a newmethod step replaces the method step 206.

During the laser step 204 in FIG. 2, there may be near completediffusion of an N-type emitter, i.e., the first emitter 318. Analternative process is described next where there is nearly no diffusionof the N-type emitter during the laser step, as shown next in the FIG.7A. This alternate process depends upon a laser profile including alaser selection, laser process variables, and the film thicknesses forthe BSG and PSG films.

Referring to FIG. 6, a flowchart 600 is shown that represents operationsin a method of forming P-type and N-type emitters for a backside contactsolar cell, in accordance with another embodiment. FIGS. 7A-7Billustrate cross-sectional views of various stages in the fabrication ofa backside contact solar cell, corresponding to operations of flowchart600, in accordance with an embodiment of the present invention. In thisexample, the mentioned process steps are performed in the order shown.In other examples, the process steps can be performed in other orders.It is to be noted that other process steps not necessary forunderstanding are omitted in the interest of clarity.

Referring to operation 602 of the flowchart 600, and to correspondingFIG. 7A, a method of forming a stack 700 (similar to FIG. 3B) of thefirst dopant material source layer 304 having P-type dopants and thesecond dopant material source layer 306 having N-type dopants above thesolar cell substrate 302 (see FIG. 3A) of the solar cell is shown.Referring to operation 604 of the flowchart 600, and to correspondingFIG. 7A, forming of a first single dopant material source layer 710containing both P-type dopants and N-type dopants sourced from both thefirst and second dopant material source layers 304, 306 (FIG. 3B) isshown. As illustrated in FIG. 7A, a third laser beam 712 may be directedto a third portion 715 of the stack 700 to form the first single dopantmaterial source layer 710.

Referring to operation 606 of the flowchart 600, and to correspondingFIG. 7B, to form a third emitter 718, the dopants of N-type may bethermally driven from the third portion 715 of the stack 700 of thefirst and second dopant material source layers 304, 306 into a thirdportion 720 of the substrate 302 underneath the first single dopantmaterial source layer 710.

Referring to operation 608 of the flowchart 600, and to correspondingFIG. 3D, to form a fourth emitter 725, the dopants of P-type may bethermally driven from a fourth portion 727 of the stack 700 of the firstand second dopant material source layers 304, 306 into a fourth portion730 of the substrate 302 adjacent to the first single dopant materialsource layer 710. Referring to operation 610 of the flowchart 600, metalcontacts may be formed to the third and fourth emitters 718, 725,respectively in the same manner as shown in FIGS. 3E-3F.

Another embodiment of forming the first and second emitters 318, 325 isshown in FIG. 8. For the ease of understanding of this embodimentrelative to the embodiment shown in FIG. 2, the step 208 of FIG. 2 isdiscussed next. The specific embodiment shown in FIG. 8 can include someor all of the method steps shown in FIGS. 2 and 3A-3F except that thethermal step 208 is replaced with a new method step as a laser step.

Referring to FIG. 8, a flowchart 800 is shown that represents operationsin a method of forming both P-type and N-type emitters for a backsidecontact solar cell by laser doping, in accordance with one embodiment.FIG. 9 illustrates a cross-sectional view of a stage in the fabricationof a backside contact solar cell, corresponding to operations offlowchart 800, in accordance with an embodiment of the presentinvention.

Referring to operation 802 of the flowchart 800, and to correspondingFIG. 9, a method of forming a P-type emitter 902, e.g., the secondemitter 325 after forming a N-type emitter 900, e.g., the first emitter318 by laser doping (see operation 204 and corresponding FIG. 3C) for abackside contact solar cell is shown. It includes directing a fourthlaser beam 912 on a fifth portion 930 of the substrate 302 adjacent tothe first single dopant material source layer 710 to diffuse the dopantsof P-type impurity conductivity type by using the fourth laser beam 912.As illustrated, a desired dopant concentration level of the P-typedopants in the fifth portion 930 of the substrate 302 may be obtained bycontrolling the configuration of the laser beam or dopant level in thefirst dopant material source layer 304 having P-type dopants (see FIG.5).

As illustrated, FIG. 9 shows a laser doping embodiment of the N-type andP-type emitters 900, 902. Referring to operation 804 of the flowchart800, metal contacts may be formed to the N-type and P-type emitters 900,902, respectively in the same manner as shown in FIGS. 3E-3F.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

1. A method of forming emitters for a back-contact solar cell, themethod comprising: forming a stack of at least two dopant materialsource layers above a substrate of the solar cell, wherein a dopantmaterial source layer of the at least two dopant material source layershas dopants of one impurity conductivity type being of opposite polarityfrom dopants of another impurity conductivity type of an adjacent dopantmaterial source layer of the at least two dopant material source layers;and directing a first laser beam on a first portion of the stack of atleast two dopant material source layers to form a single dopant materialsource layer from the at least two dopant material source layers.
 2. Themethod of claim 1, wherein directing a first laser beam on a firstportion of the stack of at least two dopant material source layersfurther comprises: patterning with the first laser beam the substrate toform a first emitter of the solar cell from the dopants of one impurityconductivity type that are available in the single dopant materialsource layer.
 3. The method of claim 2, wherein patterning with thefirst laser beam the substrate to form a first emitter of the solar cellfurther comprises: diffusing the dopants of one impurity conductivitytype with the first laser beam into a first portion of the substrateunder the single dopant material source layer at a first dopantconcentration level.
 4. The method of claim 2, further comprising:directing a second laser beam on a region of the first portion of thestack of at least two dopant material source layers to diffuse thedopants of one impurity conductivity type at a second dopantconcentration level by using the second laser beam, wherein the seconddopant concentration level is greater than the first dopantconcentration level.
 5. The method of claim 2, further comprising:patterning the substrate to form a second emitter of the solar cell fromthe dopants of another impurity conductivity type of the stacked dopantmaterial source layer of the at least two dopant material source layersadjacent to the first emitter, wherein the second emitter is differentthan the first emitter.
 6. The method of claim 5, wherein patterning thesubstrate to form a second emitter of the solar cell further comprises:thermally driving the dopants of another impurity conductivity type froma second portion of the stack of at least two dopant material sourcelayers into a second portion of the substrate adjacent to the singledopant material source layer, wherein the second portion of the stack isdifferent than the first portion of the stack, the second portion of thesubstrate is different than the first portion of the substrate.
 7. Themethod of claim 5, wherein patterning the substrate to form a secondemitter of the solar cell further comprises: directing a second laserbeam towards a second portion of the stack of at least two dopantmaterial source layers adjacent to the single dopant material sourcelayer to diffuse the dopants of another impurity conductivity type fromthe second portion of the stack of at least two dopant material sourcelayers in a second portion of the substrate adjacent to the singledopant material source layer.
 8. The method of claim 5, furthercomprising: forming a first contact hole to the first emitter of thesolar cell and forming a first metal contact into the first contact holeto electrically connect to the first emitter; and forming a secondcontact hole to the second emitter of the solar cell and forming asecond metal contact into the second contact hole to electricallyconnect to the second emitter.
 9. The method of claim 1, whereindirecting a first laser beam on a first portion of the stack of at leasttwo dopant material source layers further comprises: generating thefirst laser beam from a laser source; applying a first laser profile ofthe first laser beam to mix dopants of both the one and another impurityconductivity types from the at least two dopant material source layersinto the single dopant material source layer; and applying a secondlaser profile of the first laser beam or a second laser beam to patternthe substrate to form a first emitter of the solar cell.
 10. The methodof claim 1, wherein forming a stack of at least two dopant materialsource layers further comprises: depositing a first dopant source filmof a first impurity conductivity type above the substrate of the solarcell and depositing a second dopant source film of a second impurityconductivity type above the first dopant source film.
 11. The method ofclaim 10, wherein depositing the first and second dopant source filmsfurther comprises: forming, by chemical vapor deposition, the firstdopant source film of the first impurity conductivity type; and forming,by chemical vapor deposition, the second dopant source film of thesecond impurity conductivity type.
 12. The method of claim 11, whereinthe first impurity conductivity type is P-type, the second impurityconductivity type is N-type, and wherein forming, by chemical vapordeposition, the first dopant source film further comprises: using boronsilicate glass (BSG) to form the first dopant source film, and whereinforming, by chemical vapor deposition, the second dopant source filmfurther comprises: using phosphorus silicate glass (PSG) to form thesecond dopant source film.
 13. The method of claim 1, furthercomprising: thermally driving the dopants of one impurity conductivitytype from the single dopant material source layer into a first portionof the substrate to form a first emitter of the solar cell.
 14. Themethod of claim 13, further comprising: thermally driving the dopants ofanother impurity conductivity type from the stack of at least two dopantmaterial source layers into a second portion of the substrate adjacentto the first portion of the substrate to form a second emitter of thesolar cell.
 15. The method of claim 14 wherein the second emitter isdifferent than the first emitter and the second portion of the substrateis different than the first portion of the substrate.
 16. A method offorming emitters for a back-contact solar cell, the method comprising:forming a first dopant material source of a first impurity conductivitytype above a substrate of a solar cell; forming a second dopant materialsource of a second impurity conductivity type above the first dopantmaterial source, wherein the first impurity conductivity type isopposite the second impurity conductivity type; and directing a laserbeam on the first and second dopant material sources to form a firstemitter of the solar cell by doping a first portion of the substratewith dopants of the second impurity conductivity type.
 17. The method ofclaim 16, further comprising: forming a second emitter of the solar celladjacent to the first emitter by driving the first impurity conductivitytype from the first and second dopant material sources into a secondportion of the substrate.
 18. A solar cell, comprising: a solar cellsubstrate; a homogenous stack of at least two dopant source materiallayers disposed over a first portion of the solar cell substrate,wherein a dopant material source layer of the at least two dopantmaterial source layers has dopants of an impurity conductivity type ofopposite polarity from dopants of an impurity conductivity type of anadjacent dopant material source layer of the at least two dopantmaterial source layers; a heterogeneous dopant source material layerformed in a second portion of the solar cell substrate, theheterogeneous dopant source material layer including a mixture of thedopants of both the different impurity conductivity types from the atleast two dopant source material layers, the second portion of the solarcell substrate is different than the first portion of the solar cellsubstrate; and a first emitter having dopants of one impurityconductivity type of the different impurity conductivity types beingdisposed under the heterogeneous dopant source material layer.
 19. Thesolar cell of claim 18, further comprising: a second emitter havingdopants of another impurity conductivity type of the different impurityconductivity types being disposed under the stack of at least two dopantsource material layers, wherein the another impurity conductivity typeis opposite to the one impurity conductivity type.
 20. The solar cell ofclaim 18, wherein a central region of the first emitter is doped with agreater dopant concentration level than a non-central region of thefirst emitter.